NATO, the most successful military alliance in history, is celebrating its 70th anniversary in London during a two-day summit of the alliance's leaders.
The 29 allies will have much to discuss on December 3-4, including NATO's political future amid signs of disunity ahead of talks.
France, Turkey, and the United States are arriving with differing visions for NATO.
U.S. President Donald Trump has questioned why Washington should defend the West and has called on NATO countries to increase defense spending.
That so far has been happening, according to Secretary-General Jens Stoltenberg, who said non-U.S. allies will have increased their defense spending by $130 billion since 2016.
Canada, Turkey, and Europe are expected to pledge $400 billion in defense spending by 2024 to further appease Trump.
Meanwhile, Turkey has been criticized for buying a sophisticated Russian air-defense system that are incompatible with NATO's systems and which prompted the United States to threaten sanctions and remove Ankara from its F-35 fighter jet program.
Washington, furthermore, has criticized Turkish President Recep Tayyip Erdogan for pursuing a policy to defeat Kurdish fighters in Syria who are considered U.S. allies.
In disagreement with mounting criticism, Ankara has been withholding support for a NATO defense plan for the Baltics and Poland that requires the approval of all 29 members.
"The message to Turkey...is we need to move forward on these response plans and it can't be held up by their own particular concerns," U.S. Defense Secretary Mark Esper told Reuters as he flew to London.
French President Emmanuel Macron last month questioned NATO's central tenet that an attack on one member is an attack against all while saying the alliance is "experiencing brain-death" over the lack of an overarching strategy.
Wariness of Russia's growing military might and expansion is on the agenda, as is a new strategy to monitor the rise in China's military activity.
NATO also will for the first time name space as a domain of warfare, in addition to air, land, sea, and cyberspace.
"We are faced with a paradox," Stoltenberg told Reuters. "Yes, we have some differences, but the reality is that we are doing more together than for many years."
